Centrifugal compressors have been widely used in pumping gaseous fluids through piping systems, especially in the transportation of natural gas through pipelines.
Experimental work both in the laboratory and with field centrifugal compressors have evidenced heretofore unexplained transient phenomena in at least two areas, (1) there is response of a compressor to pulsations from an external source which might be introduced into either the compressor suction or discharge piping and (2) the effects of compressor piping on machine surge.
Some of the more specific observed phenomena are:
(1) A centrifugal compressor can either amplify or attenuate external pulsations.
(2) Even with no positive source of pulsations in the piping system, low frequency pulsations can be experienced at levels sufficiently high to fatigue compressor internals or severely shake the piping.
(3) These pulsation problems can often be mitigated by changing the pulsation response of the compressor piping (lengths, diameters, etc.). High level pulsations have been observed at frequencies ranging from less than one Hz and approaching zero, to several hundred hertz. Frequencies are not harmonically related to and do not vary with centrifugal compressor speed.
(4) The severe pulsation frequencies normally relate to one of the major pipe resonances of the piping systems, and measurements along the piping show a strong standing wave pattern, often existing across or through the compressor.
(5) The onset, frequency, and severity of machine surge can also vary as the piping system is changed.
(6) Pulsation levels are most severe when the compressor is situated at or near a velocity maximum (pressure minimum) in the pulsation standing wave field.
(7) External pulsations can induce surge in a centrifugal compressor.
As will be seen later, it is one of the purposes of this invention to provide an analog of centrifugal compressor and its associated piping system in order that the above phenomena, as well as others, can be studied and various variables optimized to minimize the effect of pulsations and machine surge.
In accordance with this invention, a nonlinear analog is provided to be operated in such a manner that, in effect, the dynamic flow impedance characteristics of a piping system is superimposed upon the compressor curves and the combined characteristics are used to predict pulsation gain or loss and system stability and the effect of various variables upon them.